Controlled current sources ate widely used in modern circuit design. In digital circuits they can be used in the final stages of the circuit output drivers. In mixed signal circuits controlled current sources can be used in line drivers, special waveform generators, switched current circuits, digital to analog converters, and the like.
In mixed signal and pure analog applications the controlled current source is typically configured as a current steering digital to analog converter (D/A). A simplified schematic of a current steering D/A converter 100 is presented in FIG. 1. The converter 100 contains a plurality of differential switching current cells 1101-110n connected to differential output lines OUT+ and OUT−. The cells are connected to a common bias voltage BIAS, which establishes the value of the current in each cell 1101-110n. The current is switched between outputs OUT+ and OUT− by complementary digital input signals Bk and Bk˜. The cells can be made identical or can be binary weighted. In special function generators additional current ratio schemes can be used, as is known in the art.
If only a single ended output is used, the described differential architecture of FIG. 1 is often used anyway. The differential architecture is used to reduce noise generation on power and ground lines, as well as to reduce the effect of the noise on the differential signal. Additionally, the differential architecture can also be used to reduce the switching noise effect on the common bias signal. In the single ended case the unused output is connected to the power line and the respective current from that output is wasted. This additional power consumption can be acceptable in some applications (e.g., internal blocks). If used in other applications (e.g., output drivers), this wasted current leads to a significant loss of power efficiency. Therefore, the noise associated with typical single ended controlled current sources is an obstacle in achieving power savings in noise sensitive applications.
This noise problem can be better understood with the reference to the circuits illustrated in FIGS. 2A and 2B. FIGS. 2A and 2B show two designs having a plurality of single ended controlled current cells 2101-210n and 2201-220n connected to the single output OUT.
In FIG. 2A, the gate of the current controlled transistor M1 is permanently connected to the bias voltage source BIAS, while the drain is controlled by switching transistor M2. This configuration can be obtained from the circuit shown in FIG. 1 by removing one of the switching transistors.
However, the circuits of FIG. 1 and FIG. 2A have substantial differences. In the differential application of FIG. 1, the drain voltage of the current control transistor M1 never goes to ground and the transistor M1 is always in saturation mode. Due to modern metal-oxide semiconductor (MOS) processes, the parasitic drain-gate capacitance is typically very small and the voltage bouncing on the drain does not significantly penetrate to the gate node. Further, if the transistor is connected in a cascode scheme, the effect is reduced even further.
In FIG. 2A, the drain voltage of the current controlling transistor M1 approaches ground, if the switching transistor M2 is OFF. Thus, the current control transistor M1 changes between linear and saturation modes. This causes a significant change of the charge accumulated in the transistor gate-source capacitor. Using a cascode scheme does not make any difference for this case because the drain voltage of the transistor goes to ground anyway since the current is turned OFF. Accordingly, the noise penetrating to the bias voltage line is substantial and can corrupt the performance of the entire controlled current source during the transition time.
Referring to FIG. 2B, the gate voltage of the current controlled transistor M1 is switched between the ground and the bias voltage. In this case the value of the gate charge change is approximately twice that of the schematic of FIG. 2A. However, the size of the switching transistor M2 and voltage of signal Bk can be chosen in such a way that the charge introduced by the current controlled transistor M1 is partially compensated by the charge of the switching transistor M2. A partial compensation is possible during the turn ON mode. During a turn OFF mode, transistor M2 removes some charge from the bias line. Accordingly, reducing the switching noise penetration in the bias voltage signal line can improve the accuracy of the controlled single ended current sources. However, noise remains a problem in single-ended current sources.